Various means for reducing NOx and reducing fuel consumption under a high load, and curbing the generation of white smoke under a low load are developed in a diesel engine having a supercharger (hereinafter, simply abbreviated as an engine in some cases). As such means, the one in which heat exchanging means is provided between an outlet of a supercharger and a combustion chamber, the heat exchanging means is constituted of a first heat exchanger at an upstream side and a second heat exchanger at a downstream side, a first and a second media having different temperatures are used for heat exchange in the first and the second heat exchangers, and after heat exchange is carried out between these first and second media and charge air, the charge air is fed into the combustion chamber (for example, patent document 1). Here, patent document 1 refers to Japanese Patent Laid-open No. 2001-342838, and the above means is described especially on page four and FIG. 1.
In concrete, according to the means described in patent document 1, the first medium at a low temperature is used in the first heat exchanger at the upstream side, and the second medium at a high temperature is used in the second heat exchanger at the downstream side. In the situation where the engine load (hereinafter, simply called a load in some cases) is light with a low fuel injection rate and white smoke is easily generated, the charge air cooled in the first heat exchanger is heated in the second heat exchanger, and the heated charge air is fed into the combustion chamber to improve ignitability and curb the generation of white smoke. On the other hand, in the situation where the engine load is heavy with a high fuel injection rate, NOx increases and fuel consumption increases, the flow of the second heat medium supplied to the second heat exchanger is eliminated or reduced, and the charge air cooled in the first heat exchanger is fed into the combustion chamber while the air is at a low temperature, thus reducing NOx and fuel consumption.
However, when the load is shifted from a high load side to a low load side, the diesel engine generally becomes unstable in its ignitability with the fuel injection rate being reduced, irrespective of the presence or absence of a supercharger and heat exchanging means. For this reason, for example, when a diesel engine is used to drive a motor and the like, the engine has more unstable ignitability and easily generates while smoke in an engine stopping step in which the load continues to be reduced to stop the engine while the engine speed is kept constant, and the fuel injection rate continues to be reduced following this. Accordingly, the development of the diesel engine capable of curbing the generation of white smoke in such a case is desired.
The above problem is prominent especially when a fuel with low ignitability such as an emulsion fuel is used, and when the engine load is below a predetermined engine load, a non-combusted fuel increases due to reduction in ignitability, thus easily generating white smoke.
Further, with the means in patent document 1, the second medium is also supplied to the second heat exchanger when the engine load is reduced from the state in which the engine load is a high load, and the fuel injection rate is reduced. However, in this situation, the second heat exchanger is cooled with charge air at a low temperature, which circulates under the high load, and therefore the heat of the second medium is depleted by (the metal components and the like of) the second heat exchanger itself. Thus, the second heat exchanger is not sufficiently heated, and it cannot warm the charge air immediately. For this reason, for some period of time after the engine load is shifted from the high load side to the low load side, less fuel exists and the charge air at a low temperature is supplied, and therefore white smoke is generated.